Passive comb filtering techniques are known for separating chrominance and luminance components of a quadrature modulated color television picture signal. Such comb filters are typically implemented with single or multiple scan line and picture frame period delays. Since the phase of chrominance is in opposition from line to line and from frame to frame (in the NTSC signal format), the process of adding a present scanning line to a line which has been delayed by one scanning line period, or adding a frame to a frame which has been delayed by one picture frame period, results in phase cancellation elimination of the chrominance component, and resultant extraction of the luminance component from the composite video color signal. By a subtractive process the chrominance component likewise may be extracted from the composite video.
In quadrature modulated color subcarrier television signal formats, comb filtering is achieved by the process of adding information coming from a certain number of successive scanning lines. This manipulation is limited to spectral areas containing both the luminance and chrominance components (e.g. 2.3 to 4.2 Mhz in the NTSC format) by means of bandpass filters. The full bandwidth luminance information is typically obtained by addition of the inverted combed bandpass filtered component and a delay matched source component. The addition of signals coming from successive lines is carried out by taking the signal from the first line, for example, and multiplying the signal by a certain coefficient, and adding the signal from a second line as multiplied by a second certain coefficient, and adding the signal from a third line as multiplied by a third certain coefficient. In a typical situation, where the signal from line 1 is V1, the signal from line 2 is V2, and the signal from line 3 is V3, a standard comb filter arrangement is: ##EQU1## (luminance in the vicinity of the subcarrier spectral area) and ##EQU2##
In this example, the fractional values 1/4 and 1/2 are the coefficients of the comb. Similar computations apply to temporal comb filters, where V1, V2 and V3 represent signals which are either undelayed (V1) or delayed by one picture frame period (V2) or by two picture frame periods (V3).
When compared to band pass filters and traps, passive comb filters work very well for separating chrominance and luminance, due to their wide bandwidth. However, the performance of passive comb filters breaks down when changes occur between lines or frames. When such changes appear, phase cancellation (averaging) from line to line or from frame to frame of the unwanted component does not perfectly occur. Instead, artifacts such as chroma blurring and horizontal dots in the luminance at the chroma subcarrier frequently are generated by the comb filtering process and may be objectionably visible to the viewer, particularly as the bandwidth of television displays has increased to include frequencies lying well above the subcarrier frequency (3.58 Mhz in the NTSC system).
A number of proposals have been presented in the prior art for changing the comb filter structure or operation during transition conditions in an attempt to avoid the unwanted picture artifacts otherwise produced. In essence, the prior approaches have been either to alter the structure of the comb filter by on-off switching operations and/or to substitute a trap or other bandwidth limiting filter in place of the comb filter for the interval in which comb filter separation of chrominance/luminance breaks down. These prior attempts to make otherwise passive comb filters adaptive at vertical chroma transitions have not achieved a satisfactory solution to eliminate unwanted picture artifacts while maintaining high bandwidth characteristics of the comb filter.
One prior proposal is set forth in the Rossi U.S. Pat. No. 4,050,084. This patent describes a digitally implemented two scanning line delay comb filter for putting out a combed chroma component and a combed luminance component when there is no change of chrominance in the vertical direction (from line to line).
When the Rossi system detects a vertical amplitude transition in the chroma between two adjacent scan lines, Rossi's chroma comb filter system thereupon switches to a 0 V1+1/2 V2-1/2 V3 configuration for the vertical chroma transition interval at the first line thereof, and then switches to a 1/2 V1+1/2 V2+0 V3 configuration for the vertical chroma transition interval at the second line thereof. This reconfiguration Rossi comb filter is carried out in real time by manipulation between zero and one half amplitude coefficient values for the V1 and V3 terms of the comb during the transition. Thus, Rossi's progressive adaptation of the comb filter structure by coefficient manipulation enables it to avoid the chroma transition and the artifacts otherwise produced. In other words, the Rossi comb filter is adaptive in the sense that in the absence of a vertical chroma transition the output is combed on the basis of three scanning lines. When a vertical transition occurs, combing reverts to a two scanning line basis, with information being combed coming during the first line of the transition from the second and third lines, and with information being combed coming during the second line of the transition from the first and second lines.
One of the drawbacks of the Rossi system is that if there are multiple chroma transitions within three adjacent scan lines, the Rossi comb filter logic collapses, and that system switches to a notch filter for the duration of the trouble. That is to say, if V1 is different from V2 and V2 is different from V3, then the Rossi system switches from comb filter processing to a low pass filter in the luminance path and to a band pass filter (notch) in the chrominance path.
Another significant drawback of the Rossi system is that it is controlled only upon detection of changes in chroma amplitude. The control signal relied upon by the Rossi system employs the difference of the rectified chroma from line to line. If, for example, a color phase shift (change in hue) occurs between two lines and it is not accompanied by a commensurate amplitude shift, the Rossi system is not capable of switching off the comb filter, and horizontal dots appear in the luminance in one or more scan lines of the picture.
A further drawback of the Rossi system is that it makes use of hard switching between the three operational configurations. There is no proportional or gentle switching between the three modes, and the switching transitions are abrupt.
Another comb filter system in the prior art was developed by Barco Electronic n.v., Noordlaan 5 Industriezone, B-8720 Kuurne, Belgium and was included in color television decoding apparatus introduced into the United States in about 1981. The Barco system was similar to that described in the Rossi patent, in that combing was switched from three lines to two lines in a manner that attempted to skirt the chrominance vertical transition. The Barco apparatus required an additional scan period delay line in order to provide a one line period look ahead or advance warning that a vertical chroma transition was imminent. Once a transition was detected on a look ahead basis, the Barco apparatus changed the coefficients of the comb on a step function (yes or no basis) by switching from the three line-based configuration to two two-line based comb filter configurations during the line blanking interval for the lines having the detected chroma vertical transition.
One apparent drawback of the Barco approach was that an entire scan line of video information was necessarily modified as a result of switching during the horizontal blanking interval, even though only a very small portion of the scan line was subject to degradation by the presence of a vertical chroma transition. As a result, in some situations high frequency luminance components became offset spatially by one line with diagonal transitions taking on a visible raggedness or step effect.
The Barco system had the same drawback as the Rossi system in that it made use only of information in the chroma bandwidth in the vicinity of the color subcarrier in order to control the switching action.
U.S. Pat. No. 4,179,705 describes a method for switching comb filter apparatus. Essentially, the '705 patent describes a method to switch off the comb filter and replace it with a low pass filter in the luminance path and a band pass filter in the chrominance path in the presence of a vertical transition. The vertical transition was detected by looking at the differences in chroma energy on a line to line basis. While this system worked reasonably well, it had a time constant (slight delay) during which to make a control decision and suffered from the frequent situation that the chroma picture information is a very weak source of information upon which to make a decision concerning chroma processing. The slight delay in the control led to poorly defined (fuzzy or soft) chroma transitions during switching. Reducing the delay led to excessive control circuit implementation costs. Thus, the system either cost too much, or it let two or three horizontal dots get through to the display screen before the trap was switched in. Consequently, while this system worked well under test signal conditions, it proved inefficient in processing real picture signal content.
U.S. Pat. No. 4,240,105 performs the same switching operation as was described in his '705 patent, but in response to different, additional control information. The system described in the '105 patent makes use not only of the chroma difference in the vertical domain but also the low frequency luminance difference which is statistically highly correlated to simultaneous chroma transitions and which provides a much stronger, more robust signal upon which to develop a switching control signal. However, while the switching based on the luminance transition was faster than the prior approach, the process of switching to a notch filter or trap in lieu of the comb filter structure during detected chroma transitions led to visibly soft transition edges: i.e., the reduced bandwidth resulting from the trap caused the picture to lack sharpness at the chroma transition.
An adaptive technique which is an improvement over the above-identified system is described in the Faroudja U.S. Pat. No. 4,864,389 which describes a comb filter apparatus which extracts at least one of the chroma and luminance from a quadrature modified subcarrier television input. The described method implemented by the described apparatus includes the steps of delaying the input signal through a plurality of predetermined delay periods wherein the duration of each delay period is related to line scan period or picture frame period, filtering the undelayed and delayed signals so that energy components of the input signal lying in the vicinity of the chroma subcarrier frequency are passed, thereby providing bandpass filtered undelayed and delayed signals, detecting amplitude and sense of periodic transitions lying in the chroma energy passband, multiplying two of the undelayed and delayed signals by the continuously variable coefficients to produce an error correction signal. However, as will be discussed in detail later, this system requires precise components in its demodulation circuits to provide clean chrominance and luminance information.
To discuss the problems with prior art filtering arrangements in a more general context, refer to the following. FIG. 1 is a representation of the conventional process for the separation of a composite signal into its components. The composite signal is typically separated first into the luminance component (Y) and a chrominance component (C) through YC separation. Hereinafter, the separation during this process will be referred to as separation in encoded space.
Thereafter, the chrominance component C is demodulated to provide a first subcomponent (U) of the chrominance component C and a second subcomponent (V) of the chrominance component C. Typically, the subcomponent U is in the blue oriented axis of the chrominance and V is in the red oriented axis of the chrominance. In such an environment the U and V subcomponents are orthogonal to each other.
Typically, conventional comb filters address the issue of transforming the separated chrominance component C into the U and V components. However, because conventional comb filters operate within the encoded space that is, before the chrominance has been demodulated, these types of systems do not provide "clean" Y and C components. That is, these conventional comb filtering techniques do not provide a luminance component without the artifacts of the chrominance or vice versa, unless the signal in two adjacent lines are either perfectly in phase or perfectly out of phase.
Since the separation of the chrominance is performed in the encoded space, the cancellation effect of the chrominance information has an inherent phase (hue) sensitivity relative to its simultaneously demodulated U and V subcomponents. One way to address this relative phase sensitivity problem is to utilize very precise binning of the phase adaptive decision logic as above described in U.S. Pat. No. 4,864,389, in the demodulation circuits that provide the U and V subcomponents to ensure that the resulting U and V subcomponents are clean. In addition, this type of system, although being an improvement over other known systems, depends on the high frequency sidebands having a precise and constant relationship with the long term phase across the line. Consequently, the solution described in the above-identified patent can prohibitively increase the cost and complexity of the comb filter such that the system would be too expensive for general applications.
Consequently, in conventional composite spectral signal environments, such as in color television environments, artifacts in the chrominance subcomponents remain when utilizing conventional comb filtering techniques. In addition, the techniques typically utilized to minimize the artifacts are complex as well as being too expensive to effectively implement.
In addition, it is desirable to scale the video image. Scaling is the ability to enlarge or shrink the video image. It is important for zoom, pan, format conversion, aspect ratio changes, optical correction, coordinate conversion, visual effects and picture-in-picture display. Although video scaling can be performed in the composite domain, it places significant restrictions on the sample selection due to the modulation of the signal. Consequently, virtually all scaling is done in the component domain.
Accordingly, what is desired is a system and method in which chrominance and luminance can be separated in an efficient manner for subcarrier spectral environments, particularly color television systems. A system and method is also desired in which the subcomponents of the chrominance can be separated in such a manner that the effects of the artifacts of one on the other can be attenuated. The method and apparatus should be easily implemented and adaptable to existing color television sets. In addition, the method and apparatus should be cost effective and easily manufactured using existing technologies. The present invention addresses such a need.